Coated phosphor filler and a method of forming the coated phosphor filler

ABSTRACT

An improved coated phosphor filler for an optical device includes a plurality of individual phosphor filler particles, and a coating layer having a plastic substance coated on each of the phosphor filler particles.

REFERENCE TO RELATED APPLICATIONS

This Application is a Division of an earlier filed application Ser. No.10/282,859 of the same title, filed Oct. 29. 2002. now issued as U.S.Pat. No. 6,734,466 and assigned to Agilent Technologies, Inc.

The invention relates to a coated phosphor filler for an optical device(e.g., a light emitting diode (LED)), a method for forming a coatedphosphor filler and a method for forming a LED making use of that coatedphosphor filler.

BACKGROUND OF THE INVENTION

In the prior art, phosphor fillers including phosphor particles are usedin a broad field of applications, ranging from electro-luminescence tophoto-luminescence devices. This rich applicability is due to thefavourable physical properties of phosphor such as a high luminescenceefficiency and lifetime as well as due to the presence of suitableemission colours in the optical emission spectrum.

A technological application of such phosphor fillers with increasingimportance is the light emitting diode (LED), which comprises a LED-chipbeing electrically connected to a electrically conducting contact base.The LED-chip usually comprises a semiconducting p-n-junction, in whichelectrons and holes, which have been injected via a supply voltage,recombine under light emission. In order to direct the light emissioninto the operational direction of the LED-Chip, the LED-Chip is usuallyencapsulated by an optical dome made of transparent resin which, inturn, may include a phosphor filler by which the light emitting spectrumof the LED-Chip can be converted as necessary.

In particular, due to the development of blue emitting LED-chips and theuse of such phosphor fillers, LED-devices providing a wide colour rangecan be obtained, including the so-called “white LED”, which can competewith conventional types of light sources in a broad field ofapplications such as traffic lights and signboards.

Generally speaking, such phosphor fillers may be based on differenttypes of phosphor compounds, namely stable and unstable phosphorcompounds. Stable phosphor compounds may e.g. comprise members of thegarnet family, preferably (YGd)₃Al₅O₁₂ including Ce³⁺-impurities.Unstable phosphor compounds may e.g. comprise SrGa₂S₄:Eu²⁺, SrS:Eu²⁺,(Sr,Ca)S:Eu²⁺, ZnS:Ag.

The advantage of a phosphor filler in the form of stable phosphorcompound particles is that it is not sensitive to moisture which would,in turn, reduce the reliability of the electrical device, like anLED-Chip encapsulated in an epoxy dome comprising such a phosphorfiller.

However, it is also known in the prior art that the performance ofdevices with unstable phosphor compound can be improved, too, by coatingthe phosphor compound material, i.e. the outer surface of the individualunstable phosphor compound particles, with a protective coating film. Inparticular, said unstable phosphor compound particles may be coated withan inorganic coating film including a moisture-proof barrier materialsuch as aluminium oxide (Al₂O₃), zinc sulphide (ZnS), silicon nitride(Si₄N₃) or the like. In case of such fillers based on unstable phosphorcompounds, the inorganic coating film on the individual-phosphorcompound particles provides for a chemical and photochemical degradationprotection of the phosphor compound.

In the light of the foregoing explanations, the term “phosphor filler”refers, in the following description, to a plurality of phosphor fillerparticles which are either stable phosphor compound particles orunstable phosphor compounds particles coated with an inorganicmoisture-proof coating film.

From U.S. Pat. No. 4,585,673, a method for forming a protective coatingfilm on unstable phosphor compound particles is known, wherein theprotective coating film is formed by a gas-phase chemical vapourdeposition (MOCVD=“metal organic chemical vapour deposition”) on thephosphor compound particles which are suspended in a fluidised bed whichis maintained in a temperature gradient, said protective coating being arefractory oxide such as aluminium oxide.

U.S. Pat. No. 6,001,477 discloses a method for providing on the surfaceof individual unstable phosphor compound particles a continuous,non-particulate coating of a metal or metalloid compound such as siliconor boron by means of a reaction between the metal or metalloid and apolymer capable of chelating ions of the metal or metalloid. Theresulting coating (e.g. a BA-PVM/MA coating) is chemically adhered tothe phosphor compound particles which exhibits improved lumenmaintenance when applied to the inner surface of a lamp envelope.

U.S. Pat. No. 5,985,175 discloses a method for providing on individualunstable phosphor compound particles a continuous, non-particulatecoating of boron oxide to enhance the quantum efficiency of the phosphorcompound particles under ultraviolet (UV) and vacuum ultraviolet (VUV)excitation. The method involves reacting a boron-containing precursorwith an oxidizing gas in a fluidised bed of phosphor particles.

Furthermore and more generally, EP 0 539 211 B1 discloses a method forproduction of a microcapsule type conductive filler, wherein thisconductive filler is dispersed in an epoxy type one-component adhesiveagent.

A possible structure of a phosphor filler 100 according to the prior artis schematically illustrated in FIG. 2 a. The phosphor filler 100comprises a plurality of unstable phosphor compound particles 101, eachof the phosphor compound particles 101 being coated with an inorganiccoating film 102. The inorganic coating film 102 consists of a suitablemoisture-proof barrier material such as e.g. aluminium oxide (Al₂O₃) andhas a thickness in the range of about 3 to 4 μm.

If the thickness of the coating film 102 is large, the coating film 102provides a significant deterioration of the optical transmissibility. Onthe other hand, if the thickness of the coating film 102 is low, thespacing between neighbouring phosphor compound particles 101 isrelatively small. Consequently, the probability of light symbolized bylight beams 103, which is e.g. emitted by a LED as described below withreference to FIG. 2 b, to be re-absorbed by surrounding phosphorcompound particles 101 is high and, therefore, the brightness obtainedin a LED using this kind of phosphor filler is low.

A typical LED 200, as schematically illustrated in FIG. 2 b, comprises aLED-chip 201, which is mounted on a first electrically conducting frame202. Said first electrically conducting frame 202 is provided with areflector cup 202 a including a recess in which the LED-chip 201 ismounted. At least two electrodes (not shown), which may be surfacemounted electrodes, are attached on said LED-chip 201, one beingelectrically connected by means of a first wiring 203 to the firstelectrically conducting frame 202, and the other being electricallyconnected by means of a second wiring 204 to a second electricallyconducting frame 205.

The LED-chip 201 is covered by a drop 206 containing a mixtureconsisting of epoxy and a phosphor filler dispersed therein, said drop206 filling almost the whole recess of the reflector cup 202 a. Thephosphor compound particles of the phosphor filler may be coated with acoating film including a moisture-proof barrier material such asaluminium oxide (Al₂O₃), i.e. they may form a structure as describedabove with respect to FIG. 2 a.

Furthermore, the major upper part of the first and second electricallyconducting frames 202 and 205 as well as the whole arrangement formed bythe LED-chip 201 covered by the drop 206 and the wirings 203 and 204 areencapsulated by an optical dome (or optical lens) 207 formed oftransparent epoxy.

The LED 200 can e.g. be operated as a white light emitting diode,wherein phosphor compound particles in the drop 206 re-emit a broad bandof yellow, yellow-green or red-green light with unabsorbed blue lightfrom the LED-chip 201.

Two common methods for forming a LED device are schematicallyillustrated in FIG. 3. These methods are generally referred to as the“pre-mix method” (FIG. 3 a) and the “pre-dep method” (FIG. 3 b).

In the so-called “pre-dep method”, as can be seen in FIG. 3 b, aLED-chip 301 of a LED device 300 is placed inside a reflector cup 302 ofa metal base 303 in a first step. Then the LED-chip 301 is electricallyconnected, by means of wirings 304, to the metal base 303. In the nextstep, a drop 305 containing a mixture of phosphor compound particles 306and epoxy 307 is filled into the reflector cup 302 to cover the LED-chip301. Finally, the whole structure of the drop 305 covering the LED-chip301, the wirings 304 and the metal base 303 is over-moulded with epoxyto form a transparent optical dome 308.

In contrast to this method, the so-called “pre-mix method” prevents aprocedure of covering of the LED-chip 301 in two steps. To achieve thissimplification of the manufacturing process, the LED-chip 301 isover-moulded, as can be seen in FIG. 3 a, in only one step by an opticaldome 309 containing a pre-mixed mixture of phosphor compound particles310 and epoxy 311.

Accordingly, whereas the pre-mix method of FIG. 3 a simplifies themanufacturing process, the pre-dip method of FIG. 3 b provides, due tothe completely transparent optical dome 308, a more efficient lightextraction from the LED-chip 301.

However, optical devices such as a light emitting diode (LED) includingphosphor fillers according to the prior art, i.e. stable or unstablephosphor compound particles being coated with none or only oneprotective coating film, respectively, wherein the coating film consistsof e.g. aluminium oxide, exhibit several shortcomings for the followingreasons:

-   -   (1) A significant basic problem of prior art LED devices of the        type described above is that the phosphor filler, i.e. the        individual phosphor compound particles tend to agglomerate. This        problem is observed and equally valid for all type of phosphor        fillers discussed above, that is for stable phosphor compound        particles as well as for unstable phosphor compound particles        which are coated with an inorganic moisture-proof coating film.        Such an agglomeration leads, however, to a number of drawbacks        in the operating characteristics of the LED device, such as        uneven spectral and brightness distribution of the emitted light        over the emitting surface of the device, loss of brightness of        the LED device based on re-absorption effects between        neighbouring agglomerated phosphor particles, etc.    -   (2) The prior art LED devices containing unstable phosphor        compound particles coated with an inorganic film as phosphor        filler exhibits a relatively poor light extraction efficiency.        In other words the amount of light emitted by such a device        compared to the amount of light which would be emitted by a        device which does not comprise such a phosphor filler is        significantly reduced. This is based on the fact that the        refractive index of the inorganic coating film, such as        aluminium oxide, differs from the refractive index of the epoxy        resin, resulting in that the light entering and passing through        the encapsulating dome experiences several times total        reflection at the inorganic coating film—epoxy—interfaces and,        thereby, captured within the dome.    -   (3) In the prior art LED device 200, the unstable phosphor        compound particles in drop 206 exhibit a relatively large        sensitivity to moisture, which may enter in the drop 206 and        attack the unstable phosphor compound particles situated        therein. Consequently, the LED 200 is subjected to aging        effects, so that the reliability of such a prior art LED is        relatively low. This effect is especially disadvantageous in        applications such as traffic lights or signboards, which        generally require lifetimes of the used optical device of more        than 10⁵ h.    -   (4) If unstable phosphor compound particles are used in the drop        206 of the LED 200 which are individually coated with a        protective coating film consisting of e.g. aluminium oxide, the        protective coating reduces the optical transmission of the drop        206 and thereby the brightness of the LED 200. Accordingly, the        thickness of this protective coating and, consequently, the        protection of the unstable phosphor compound particles are        limited.

Accordingly, the performance of LED's using prior art phosphor filler isinsufficient particularly with respect to the agglomeration problem,but, at least in case of unstable phosphor compound particles also withrespect to light extraction, i.e. the brightness of the LED.Accordingly, the reliability of the known LED devices is low.

SUMMARY OF THE INVENTION

one feature of the present invention is to provide a phosphor filler anda method for forming a phosphor filler which enable the production ofoptical devices, such as light emitting diodes (LEDs) including laserdiodes, with an improved luminous performance and reliability.

Another feature of the present invention is to provide a light emittingdiode (LED) (or a laser diode) and a method for forming a light emittingdiode (or a laser diode) with improved luminous performance andreliability.

A further feature of the invention is to provide a phosphor fillerwhich, whether it is based on stable or unstable phosphor compounds, canbe evenly dispersed in the transparent plastic substance, like epoxyresin of the optical dome of optical devices.

A further feature of the invention is to provide a phosphor filler whichcan be evenly dispersed and which, at the same time, eliminates or atleast reduces the disadvantageous effects of a thick inorganic coatingfilm on unstable phosphor compound particles.

According to one aspect of the invention, there is provided a coatedphosphor filler that includes a plurality of individual phosphor fillerparticles that are coated with a coating layer having a plasticsubstance, preferably an optically transparent epoxy composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are schematic illustrations showing preferredembodiments of the coated phosphor filler according to the presentinvention and its effect on the ability of light emission.

FIGS. 2 a-2 b are a schematic illustration of a phosphor filleraccording to the prior art (FIG. 2 a) and a schematic illustration of alight emission diode (LED) according to the prior art (FIG. 2 b); and

FIGS. 3 a and 3 b show alternative embodiments of a method for forming alight emission diode (LED).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As will be described in more detail, a coated phosphor filler includes aplurality of individual phosphor filler particles that are coated with acoating layer having a plastic substance, preferably an opticallytransparent epoxy composition.

Due to this structure of the coated phosphor filler, the performance ofan optical device, such as a LED, using such a coated phosphor filler issignificantly improved with respect to light extraction, i.e. brightnessof the LED device, and reliability of the LED device. Another advantageis that an enhanced reliability of the obtained LED is achieved due to abetter passivation of the individual phosphor filler particles againstelevated temperatures and humidity.

Since with regard to the optical transmissibility of the phosphorfiller, the thickness of the coating layer made in accordance with oneembodiment of the present invention is not critical, the distancebetween neighbouring phosphor filler particles may be significantlyenhanced without a deterioration of the optical transmissibility orlight extraction. Consequently, the coating film according to thepresent invention may prevent an agglomeration between neighbouringphosphor particles which is a fundamental advantage of the coatedphosphor filler according to the invention. Further, re-absorptioneffects between different phosphor filler particles are prevented andthe luminous performance of the LED device is improved by the use of theinventive coated phosphor filler.

Furthermore, a significant improvement of the light extraction can beachieved with an optical device using the coated phosphor filler madeaccording to the invention. On the one hand, this effect is achieved dueto a modification of the refractive indices present between theindividual phosphor filler particles and the outer epoxy encapsulation.The inventive structure of the coated phosphor filler is particularlyadvantageous because the coating layer includes the plastic substance,in particular if including an optically transparent epoxy composition,provides additional “interfaces” between the individual phosphor fillerparticles and the outer epoxy encapsulation. This “interfaces”,preferably epoxy-epoxy-interfaces, have the effect to enhance theextraction efficiency of the optical device due to a modification of therefractive index, since such a modification of the refractive indexresults in a reduction of Fresnel reflection losses which mightotherwise be significant, since the refractive index of e.g. the usedLED-chip is relatively high compared to the refractive index of thesurrounding epoxy composition. Accordingly, with the presence ofadditional epoxy-epoxy-interfaces between the LED-chip and thesurrounding epoxy encapsulation, these Fresnel reflection losses will bereduced and the light extraction will be enhanced.

On the other hand, a significant improvement of the light extraction inan optical device using the inventive coated phosphor filler is also dueto the fact that the need of thick protective layers consisting of e.g.an inorganic passivation material is eliminated. Such protective layersbeing coated on the individual phosphor particles result in asignificant deterioration of light extraction, since they absorb asignificant part of the light being emitted by e.g. the LED chip. In thepresent invention, such a protective passivation layer is either totallydispensable (in case that the phosphor filler particles are stablephosphor compound particles), or the thickness of such a passivationlayer may be significantly reduced (in case the phosphor fillerparticles are unstable phosphor compound particles and an additional,but relatively thin barrier film is provided between the coating layerand the individual phosphor filler particles).

According to a preferred embodiment, the inventive coated phosphorfiller structure is applied to phosphor filler particles which areformed by unstable phosphor compound particles coated with a moistureproof barrier film. In this case the coating layer comprising theoptically transparent epoxy composition is coated on the barrier film.The phosphor compound particles may e.g. comprise at least one of thecomponents SrGa₂S₄:Eu²⁺, SrS:Eu²⁺, (Sr,Ca)S:Eu²⁺ and ZnS:Ag.

In the structure of the coated phosphor filler according to thispreferred embodiment, the basic aim of the barrier film is to protectthe unstable individual phosphor compound particles from aging effectscaused by the surrounding environment such as moisture, therebypreventing any change in the chemical composition of the unstablephosphor compound particles and preserving their quantum efficiency.

However, according to another preferred embodiment, the inventive coatedphosphor filler structure can also be applied to phosphor fillerparticles which are formed by stable phosphor compound particles whichdo not need to be coated with a moisture proof barrier film. In thiscase, the phosphor filler particles may comprise at least one member ofthe garnet family, preferably (YGd)₃Al₅O₁₂ including Ce³⁺-impurities.

The basic aim of the coating layer is to prevent an agglomeration of thephosphor filler particles (formed by either one of the aboveembodiments) and to improve the light extraction to enhance thebrightness of the LED device, and also to protect the barrier filmagainst any chemical decomposition effects. Furthermore, the coatinglayer also provides for additional interfaces between the individualphosphor filler particles and the outer epoxy encapsulation as alreadyexplained above, thereby also contributing to an improvement of thelight extraction and the brightness of the optical device.

Said barrier film is preferably formed of an inorganic passivationmaterial, which may include a material selected from the groupconsisting of aluminium oxide, silicon monoxide, zinc sulphide orsilicon nitride.

The thickness of the coating layer is preferably in the range of 2 to 6μm, more preferably 3 to 5 μm.

The thickness of the moisture-proof barrier film is preferably in therange of 0.1 to 2 μm. In particular, the thickness of said coating layeris at least twice the thickness of said barrier film. This is particularadvantageous, since due to the optical transparency of the coatinglayer, the thickness of said coating layer is less critical than thethickness of the barrier film, the latter absorbing a relatively largeamount of light emitted by e.g. the LED chip. On the other hand, thelarge thickness of the coating layer provides for an effectiveprotection of the individual phosphor filler particles againstagglomeration.

The thickness of said coating layer may also be 2 to 10 times thethickness of said barrier film.

Said phosphor filler particles preferably have a sphere-like shape,resulting in a relatively easy preparation procedure. Furthermore, sucha shape represents an optimum optical and geometrical structure andmakes high packing densities in very thin coating layers with relativelylow light-scattering possible.

Said epoxy composition is preferably containing hydrophobic residuesforming a moisture-repellent barrier, providing for an additionalmoisture protection of the individual phosphor filler particles.

According to another aspect of the invention, in a method for forming acoated phosphor filler, in particular for use in light emitting diodes,comprising a plurality of individual phosphor filler particles, saidphosphor filler particles are coated with a coating layer comprising aplastic substance.

According to a preferred embodiment, unstable phosphor compoundparticles are used as said phosphor filler particles and said step ofcoating said phosphor filler particles with a coating layer furthercomprises the steps of coating said unstable phosphor compound particleswith a moisture proof barrier film, and coating the outer surface ofsaid moisture proof barrier film with said coating layer.

According to a preferred embodiment, said step of coating said unstablephosphor compound particles with a moisture proof barrier film isperformed by chemically forming said moisture proof barrier film insolution. Said step of coating the outer surface of said moisture proofbarrier film with said coating layer may be performed by physicallydepositing said coating layer on said moisture proof barrier film.

According to another aspect of the invention, a method for forming alight emitting diode (LED) comprises the steps of mounting a LED-chip ona contact base, electrically connecting said LED-chip to a first and asecond electrically conducting frame, covering said LED-chip with acoated phosphor filler, said coated phosphor filler including aplurality of individual phosphor filler particles, wherein said coatedphosphor filler is subjected to a pre-treatment, wherein said phosphorfiller particles are coated with a coating layer comprising a plasticsubstance.

According to another preferred embodiment, unstable phosphor compoundparticles are used as said phosphor filler particles and said step ofcoating said phosphor filler particles with a coating layer furthercomprises the steps of coating said unstable phosphor compound particleswith a moisture proof barrier film, and coating the outer surface ofsaid moisture proof barrier film with said coating layer.

According to a further preferred embodiment, the step of covering saidLED-chip in said first electrically conducting frame with a coatedphosphor filler further comprises the steps of dispensing said LED-chipwith a drop of said coated phosphor filler in a reflector cup providedin said first electrically conducting frame, and over-moulding said dropand at least a part of said first electrically conducting frame with anoptical dome consisting of an optically transparent epoxy.

According to another preferred embodiment, the step of covering saidLED-chip in said first electrically conducting frame with a coatedphosphor filler further comprises the steps of forming a mixture betweena plurality of said individual phosphor filler particles and anoptically transparent epoxy, and over-moulding said LED-chip and atleast a part of said first electrically conducting frame with saidmixture to form an optical dome.

According to a further aspect of the invention, a mixture is providedwhich is prepared in particular for use in the above method. Saidmixture is a mixture of a plurality of said individual phosphor fillerparticles and an optically transparent epoxy, wherein said phosphorfiller particles are coated with a coating layer comprising a plasticsubstance and is advantageous in so far, as the necessary compound doesnot have to be individually prepared in each manufacturing process toobtain the above desired effects according to the present invention.

According to another aspect of the invention, a light emitting diode(LED) comprises a LED-chip mounted on a contact base, said LED-chipbeing electrically connected to a first and a second electricallyconducting frame, and a coated phosphor filler, said coated phosphorfiller including a plurality of phosphor filler particles and coveringsaid LED-chip, wherein said phosphor filler particles are coated with acoating layer comprising a plastic substance.

According to a preferred embodiment, said LED-chip is covered with adrop of said coated phosphor filler in a reflector cup provided in saidfirst electrically conducting frame, and said drop and at least a partof said first electrically conducting frame are over-moulded with anoptical dome consisting of an optically transparent epoxy.

According to another preferred embodiment, said LED-chip and at least apart of said first electrically conducting frame are over-moulded with amixture between a plurality of said individual phosphor filler particlesand an optically transparent epoxy, said mixture forming an opticaldome.

According to another preferred embodiment, an optical dome is providedcovering said LED-chip and consisting of an epoxy material, providingfor a protection of the whole arrangement without disturbing its opticalproperties.

FIG. 1 a and FIG. 1 b demonstrate in a schematic view the effect of theinventive structure of the coated phosphor composition on the ability oflight emission.

FIG. 1 a shows a preferred embodiment of a coated phosphor filler 10according to the present invention. The coated phosphor filler 10includes a plurality of stable phosphor compound particles 11, which mayinclude at least one member of the garnet family, preferably(YGd)₃Al₅O₁₂ including Ce³⁺-impurities.

The individual phosphor compound particles 11 have a sphere-like shape,and each phosphor compound particle has a diameter in the range of 10±5μm. Each of the phosphor compound particles 11 is coated with a coatinglayer 12 comprising a plastic substance, preferably an opticaltransparent epoxy. According to this preferred embodiment of theinvention, the coating film is 3 to 4 μm thick.

Preferably, the coating layer 12 is coated on the individual phosphorparticles by physically depositing the coating film on the individualphosphor compound particles. This may e.g. be performed by an immersionof the phosphor compound particles in the epoxy, followed by a dryingperiod.

The epoxy composition forming the coating layer 12 preferably containshydrophobic residues, which constitute an additional moisture-repellentbarrier and, thereby, provide for an additional protection of the stablephosphor compound particles 11.

Experiments have shown that phosphor fillers coated with a coating filmaccording to the invention do not tend to agglomerate when the phosphorfiller particles are dispersed in the epoxy used e.g. to form atransparent optical dome above a LED-chip.

Furthermore, since the coating layer 12 is formed of a transparent epoxymaterial, it does not deteriorate the light extraction of the opticaldevice, e.g. the LED-chip. As can be seen in FIG. 1 a, light beamssymbolized by arrows 13 are extracted from a particular phosphorparticle without being disturbed by the surrounding phosphor particles11 or by the coating film 12. Consequently, the brightness of an opticaldevice, such as a LED, using the phosphor filler according to theinvention 10 is enhanced.

FIG. 1 b shows another preferred embodiment of a coated phosphor filler20 according to the invention. In this embodiment, the coated phosphorfiller includes a plurality of unstable phosphor compound particles 21,which may e.g. comprise at least one of the components SrGa₂S₄:Eu²⁺,SrS:Eu²⁺, (Sr,Ca)S:Eu²⁺ and ZnS:Ag.

Each individual phosphor compound particle is coated with a film 22consisting of a suitable moisture-proof barrier material, preferably aninorganic passivation material such as e.g. aluminium oxide (Al₂O₃).Said barrier material may also contain other types of passivationmaterials such as silicon monoxide (SiO), zinc sulphide or siliconnitride (Si₃N₄).

The coating of the unstable phosphor compound particles with themoisture proof barrier film 22 is preferably performed by the so-calledWet Chemical process. Again, the coating layer 23 may be coated on themoisture proof barrier film 22 by physically depositing the coating filmon the moisture proof barrier film 22, which may e.g. be performed by animmersion of the phosphor compound particles coated with the film 22 inepoxy and following by a drying period.

According to the invention, on each moisture-proof barrier film 22 acoating layer 23 is coated, which corresponds to the coating layer 12 inthe coated phosphor filler 10 of FIG. 1 a, and which comprises a plasticsubstance, preferably an optical transparent epoxy. In a preferredembodiment, oxygen-containing functional groups of the epoxy compoundschemically interact, i.e. form a chemical bond, with the metal ions(e.g. aluminium ions) of the barrier material (e.g. aluminium oxide,Al₂O₃). Moreover, the coating layer 23 is again preferably formed byhydrophobic epoxy compounds which constitute an additionalmoisture-repellent barrier providing for an additional protection of theunstable phosphor compound particles 21. Consequently, according to thispreferred embodiment of the invention, the thickness of the barrier film22 can be reduced compared to the thickness of the barrier film 102 ofthe prior art phosphor filler 100 as shown in FIG. 2 a withoutdeteriorating the moisture protection of the unstable phosphor compoundparticles 21. Further, an optical device such as a LED having an opticaldome with phosphor filler particles dispersed therein, wherein thephosphor filler particles comprise a thin barrier film according to thispreferred embodiment of the invention, provides for superior lightemitting characteristics due to the thinner moisture-proof barrier film.

Preferably, the thickness of said coating layer 23 is relatively largecompared to the thickness of said barrier film 22. More specifically,the thickness of the coating layer 23 can be at least twice, orpreferably 2 to 10 times the thickness of the barrier film 22.

In particular, the thickness of the moisture-proof barrier film 22 canbe in the range of about 0.1 to 2 μm, whereas the coating layer 23 mayhave a thickness of about 2 to 6 μm and, further preferably, of 3 to 5μm.

Similar to the embodiment shown in FIG. 1 a, the phosphor fillercomprising unstable phosphor compound particles 21 coated with themoisture-proof barrier film 22, which, in turn, are coated with thecoating layer according to this embodiment of the invention shown inFIG. 1 b, does not tend to agglomerate when the phosphor fillerparticles are dispersed in the epoxy used e.g. to form a transparentoptical dome above a LED-chip.

As can be seen from FIG. 1 a and FIG. 1 b, the distance betweenneighbouring phosphor filler particles 11 or 21 and 22 in the coatedphosphor filler 10 and 20, respectively, is significantly enhancedcompared to the prior art phosphor filler illustrated in FIG. 2 a.Consequently, the ratio of light (symbolized in FIG. 1 a and FIG. 1 b bylight beams 13 or 24, respectively) emitted through the gap which isleft between surrounding phosphor filler particles 11 or 21 and 22 isenhanced, leading to an improvement of the brightness achieved in a LEDusing this kind of coated phosphor fillers.

Since the coated phosphor filler 10 of FIG. 1 a comprises stablephosphor compound particles 11 and, therefore, does not need amoisture-proof barrier film around the particles 11, the phosphor filler10 exhibits a higher optical transmissibility compared to the phosphorfiller 20 of FIG. 1 b which comprises unstable phosphor compoundparticles 21 coated with the moisture-proof barrier film 22. However, inthe phosphor filler 20 of FIG. 1 b, the thickness of the additionalbarrier film 22 including the moisture-proof barrier material may besignificantly lower than in the prior art, while still giving anadditional protection to the unstable phosphor compound particles 21against humidity and corresponding aging effects by means of the coatinglayer 23.

The coated phosphor filler according to the present invention may beused to form optical devices, such as a white LED as illustrated in FIG.2 b, by means of conventional methods, such as the “pre-mix” method orthe “pre-dep” method described above with respect to FIGS. 3 a,b. Due tothe inventive structure of the coated phosphor filler, the performanceof such an optical device is significantly improved with respect tolight extraction and brightness of the optical device, but also withrespect to the reliability of the optical device.

1. A coated phosphor filler, comprising: a plurality of individualphosphor filler particles; and a coating layer coated on the phosphorfiller particles, wherein the coating layer comprises a plasticsubstance, wherein the phosphor filler particles are unstable phosphorcompound particles coated with a moisture-proof barrier film, thecoating layer being provided on the outer surface of said barrier film,wherein the thickness of the coating layer is at least twice thethickness of said barrier film.
 2. The coated phosphor filler accordingto claim 1, wherein the plastic substance comprises an opticallytransparent epoxy composition.
 3. The coated phosphor filler accordingto claim 1, wherein the phosphor compound particles comprise at leastone of the components SrGa₂S₄:Eu²⁺, SrS:Eu²⁺, (Sr,Ca)S:Eu²⁺, and ZnS:Ag.4. The coated phosphor filler according to claim 1, wherein said barrierfilm is formed of an inorganic passivation material.
 5. The coatedphosphor filler according to claim 4, wherein said inorganic passivationmaterial includes a material selected from the group consisting ofaluminum oxide, silicon monoxide, zinc sulphide and silicon nitride. 6.The coated phosphor filler according to claim 1, wherein the thicknessof the coating layer is in the range of 2 to 6 μm.
 7. The coatedphosphor filler according to claim 1, wherein the thickness of themoisture-proof barrier film is in the range of 0.1 to 2 μm.
 8. Thecoated phosphor filler according to claim 1, wherein the thickness ofsaid coating layer is 2 to 10 times the thickness of said barrier film.9. The coated phosphor filler according to claim 2, wherein said epoxycomposition includes hydrophobic residues forming a moisture-repellentbarrier.
 10. A method for forming a coated phosphor filler, said methodcomprising: coating each of a plurality of individual phosphor fillerparticles with a moisture proof barrier film; and coating the outersurface of said moisture proof barrier film with a coating layercomprising a plastic substance, wherein the thickness of the coatinglayer is at least twice the thickness of the moisture proof barrierfilm.
 11. The method according to claim 10, wherein said step of coatingeach of said plurality of individual phosphor filler particles isperformed by using the Wet Chemical process.
 12. The method according toclaim 11, wherein said step of coating the outer surface of saidmoisture proof barrier film with said coating layer is performed byphysically depositing said coating layer on said moisture proof barrierfilm.
 13. The method according to claim 10, wherein an inorganicpassivation material is used as said barrier film.
 14. A coated phosphorfiller, comprising: a plurality of stable phosphor filler particles,wherein the stable phosphor filler particles comprise at least onemember of the garnet phosphor family; and a coating layer coated on thephosphor filler particles, wherein the coating layer comprises a plasticsubstance, wherein the plastic substance comprises an opticallytransparent epoxy composition.
 15. The coated phosphor filler accordingto claim 14, wherein the stable phosphor filler particles comprise(YGd)₃A₁₅O₁₂ including Ce³⁺-impurities.